1. Field of the Invention
The present invention relates to a thickness measurement apparatus and a thickness measurement method, and in particular relates to a thickness measurement apparatus and a thickness measurement method applied to measurement of a thickness of a thin measured object that is highly transparent, such as a transparent film.
2. Description of Related Art
Conventionally, a confocal point displacement gauge, chromatic confocal-type displacement gauge, spectral interference-type thickness gauge, and the like are known as devices conducting non-contact measurement of the thickness of a thin, plane parallel plate-shaped transparent measured object, such as a film or sheet.
For example, in a confocal point displacement gauge, a focal position of light fired at a measured object is changed, and a position of the measured object is detected using the focal position where an amount of reflected light passing through a pinhole provided at a confocal point position is greatest.
In addition, in a chromatic confocal-type displacement gauge, light from a white light source is fired (i.e., emitted) at a measured object using a field lens having significant axial chromatic aberration, and the most intense wavelength in the reflected light passing through a pinhole provided at the confocal point position is detected, thereby detecting a position of the measured object. Japanese Patent Laid-open Publication No. 2011-39026 discloses a chromatic confocal-type displacement gauge having high resolution and high-speed response.
In a spectral interference-type thickness gauge, light from a white light source is fired at a measured object, and light reflected off a front surface of the measured object and light reflected off a back surface of the measured object scatter and then are caused to interfere with each other; thereby, an intensity pattern of light interference with respect to light wavelength is obtained and a position of the measured object is detected.
However, in the conventional confocal point displacement gauge and the chromatic confocal-type displacement gauge, in a case where the measured object is a thin transparent body having a thickness of 10 μm or less, for example, the light reflected by the front surface of the transparent body is difficult to distinguish from the light reflected by the back surface of the transparent body because the two lights are too close together, and a thickness measurement cannot be made with a high degree of accuracy.
In addition, in the spectral interference-type thickness gauge, although a thin transparent body having a thickness of 10 μm or less can be measured, measurement of a thick transparent body of 1 mm or more is difficult. This is because the intensity pattern of the light interference with respect to the thick transparent body becomes too narrow and detection with an image sensor or the like becomes difficult. In addition, responsiveness may be problematic. In other words, in the spectral interference-type thickness gauge, all pixels of the image sensor are photoreception pixels, and therefore detection of an amount of light received takes time. Moreover, data for a massive amount of light received by all pixels of the image sensor is analyzed by FFT (Fast Fourier Transform) and the thickness of the measured object is calculated. Therefore, calculation time may become extended.
Further, in the conventional technologies, a size of a measurement spot is determined by design specifications, and therefore the size of the measurement spot cannot be changed at the time of measurement.